Equipment and methods for emergency lighting that provides brownout detection and protection

ABSTRACT

The present invention relates to systems, equipment and methods that provide emergency lighting and allow the detection of brownout conditions. One aspect of the present invention is an emergency lighting system with an input voltage interface for receiving an input voltage, a brownout detection component for detecting a brownout condition on the input voltage and generating a brownout signal, a switch mode power converter for altering the input voltage; and an emergency lighting control and battery charging component for controlling the charging of a battery pack and receiving the brownout signal. Another aspect of the invention is a method of providing emergency lighting performed by receiving an input voltage; automatically establishing a brownout threshold relative to the input voltage; and detecting a brownout condition.

RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 10/952,013, filed Sep. 28, 2004, now pending, the entire contents ofwhich are incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to emergency lighting systems that providebrownout detection and protection. Specifically, the invention relatesto emergency lighting units capable of detecting a brownout conditionthat occurs when there is a reduction of normal AC line voltage.

BACKGROUND OF THE INVENTION

Emergency lighting systems provide necessary light and protect againstthe dangerous conditions that may exist due to the lack of adequatelighting during a lighting emergency. Such a lighting emergency mayoccur when there is an interference with the normal electricity providedin a building or to the building's surroundings. In lighting emergenciesit is often beneficial to illuminate specific areas, signs, exitsignals, walkways, stairways, or to otherwise allow for the use of andexit from the premises by providing light or signals. Emergency lightingsystems detect a lighting emergency and allow for automatic and adequateillumination when such an emergency occurs.

An emergency lighting system is one or more device, unit, apparatus,equipment or component used to detect the existence of a lightingemergency and/or provide emergency lighting or power for emergencylighting in a lighting emergency. An emergency lighting system mayinclude one or more separately placed emergency detection and emergencylighting units.

Some emergency lighting systems detect the existence of an emergencycondition by monitoring the electricity supplied to a building, or aportion of a building, and detecting when the electrical conditions areinadequate to provide for normal lighting. Note that the use ofemergency lighting systems is not restricted to buildings. Emergencylighting is generally applicable anywhere that lighting or electricityis used. In that respect, the technology surrounding emergency lightingsystems has far reaching applications, including potential uses inelectronic devices, computers, automobile electronics, aircraftelectronics, and many others.

Many emergency lighting systems are capable of detecting electricalconditions that are inadequate to provide for normal lighting. One suchcondition, known as a blackout, occurs a when a building, portion of abuilding, or other area loses power. Generally, such a condition occurswhen the external power being supplied, is substantially or completelyshut off, when there is a power outage, power failure, or othersignificant power disturbance. During a blackout, an emergency lightingsystem may detect and respond to the emergency condition by illuminatingemergency lights and signs.

A brownout is another type of emergency lighting condition. A brownoutoccurs when there is a reduction of normal voltage in the electricityprovided to a building, or portion of a building. For example, abrownout may occur on a summer day when the demand for electricity froma given power company is particularly high causing a drop in the voltagelevel provided by that power company. The voltage level may drop to thepoint where some or all of the normal lighting circuits cease tofunction.

Some emergency lighting systems utilize brownout detection circuitry todetect a reduction in the voltage that is supplied to the normallighting system. The brownout detection circuitry is generally intendedto recognize a situation when the voltage is too low for normal lightingcircuits to provide illumination for a space. Note that some brownoutdetection circuitry may also detect a blackout since during a blackoutthere is little or no voltage or power. In many systems, once thebrownout detection circuitry makes a determination that the voltage istoo low, a battery powered lamp is turned on to provide lighting untilthe voltage rises to a point sufficient to resume providing illuminationthrough the normal lighting system. The emergency lamps are usuallydirect current (“DC”) powered lamps intended to efficiently provide anadequate level of illumination to ensure safety during the poweremergency.

A building's normal electricity supply provides an input voltage thatmay be both monitored and used by an emergency lighting system.Emergency lighting systems use the input voltage, usually afterconverting the voltage to direct current, to provide power to charge orrecharge a battery. The systems also monitor the input voltage foremergency lighting condition such as a blackout or a brownout.Accordingly, many emergency lighting systems include (1) poweringcircuitry to use the input voltage to power the charger and (2)detection circuitry to monitor the input voltage for the existence of anemergency condition.

Prior art methods of powering the charger and detection circuitry inemergency lighting systems typically have involved either a transformeror capacitive input circuit. These components were used to change thevoltage level or otherwise alter the electricity provided by the inputvoltage to an appropriate type useful to the battery charger ordetection circuitry.

A transformer is an energy coupling device that takes electrical energyat one voltage and transforms it to another voltage. The new voltage maybe higher (stepped up) or lower (stepped down), or it may remain thesame as the input voltage. For example, if an input voltage of 480V, therated voltage, is applied to the primary of a 480V-120V single windingtransformer, the secondary voltage produced by the transformer will be120V. In use, however, the input voltages are often higher or lower thanthe rated voltage of a transformer's primary. In these instances thesecondary voltage will be higher or lower respectively. For example, a480-120V single winding transformer with an input line voltage of 456Vwill have a secondary output voltage of 114V. This is because thetransformers voltage ratio is 4:1 (480V primary divided by 120Vsecondary). Thus, its secondary voltage is 456V divided by 4, or 114V.Conversely, this same transformer with an input voltage of 504V willhave a secondary voltage of 126V (504V divided by 4).

Transformers often have one or more voltage taps. A voltage tap is anadditional connection on either the primary or secondary side of thetransformer. A voltage tap allows the user of the transformer to alterthe transformer's voltage ratio. As described above, the voltage ratiodetermines the voltage transformation that takes place. There are timeswhen the actual incoming voltage is different than the expected normalincoming voltage. When this happens, it may be advantageous to be ableto change the voltage ratio in order to get the desired (rated) outputvoltage. Voltage taps, designed into the transformer's primary, deliverthis desired flexibility. In other words, tapping the primary in anumber of different spots provides a means to adjust the turns ratio andfine-tune the secondary output voltage. These tap connections areusually set at the factory for normal line voltages. Duringinstallation, the appropriate tap may be selected depending on the inputvoltage present at the installation site.

In emergency lighting systems, transformers have been used to step downan input voltage to a lower voltage, which is then used to power thecharger circuitry. Because the transformer could have multiple inputvoltage taps, the transformer could accept input voltages of variousmagnitudes allowing the emergency lighting system to be used indifferent voltage environments. For example, one common method has beento utilize a 60 Hz line rated transformer with taps for 120 and 277 VAC.During installation the electrician could select the appropriate tap forthe voltage level at the site.

Capacitive divider circuits are also used to step down an input voltageto power the charger circuitry in emergency lighting systems. Liketransformers, capacitive divider circuits can also have taps, whichallow the use of emergency lighting systems using these circuits indifferent voltage environments. For example, a capacitive dividercircuit with taps for 120 and 277 VAC could be used in an emergencylighting system.

The use of a transformer or capacitive divider circuit in past emergencylighting systems allowed for relatively simple brownout detectioncircuitry in those systems. There were two main categories of brownoutdetection circuits used in these systems. Some brownout circuitsutilized a set input voltage tap on the primary side of the transformeror capacitive divider while other brownout detection circuits used thesecondary voltage produced on the secondary side of the transformer orcapacitive divider.

In the first category of brownout detection circuits mentioned above,the circuit took advantage of the availability of a set voltage tap,usually 120V, present on in the emergency lighting system. The abovemethods of powering the charger circuitry with a transformer ofcapacitive divider circuit insured that no matter what voltage wasapplied to the system in the field, a set voltage tap, usually 120V, ofthe transformer or capacitive divider always had a set voltage presentthat varied proportionally with the input voltage. Because this was thecase, a simple circuit could be used to generate a DC voltage that wasthe same regardless of the input tap selected and in proportion to theincoming AC input voltage regardless of the input voltage level. Theability to generate a single DC voltage proportional to the inputvoltage level regardless of the input voltage level meant that a simplecomparator circuit could be used to detect a brownout condition bydetecting drops in the input voltage.

In the second category of brownout detection circuits mentioned above,the circuit utilized the secondary voltage produced on the secondaryside of the transformer or capacitive divider. These circuits use areduction in the voltage on the secondary side of the transformer orcapacitive divider to infer a reduction in the input voltage on theprimary side.

Although detecting brownout conditions on the secondary side worksreasonably well, this method has significant disadvantages.Specifically, the loading of the charger circuitry by a dischargedbattery can be mistaken for a reduction in the input voltage. Because ofthis disadvantage, circuits that employ this method for detecting abrownout condition normally have the point where the brownout circuitturns on the emergency circuit, the brownout threshold, preset at asignificantly lower percentage of normal input voltage (eg. 65-70%) toavoid false triggers of the brownout circuit under conditions of heavytransformer loading. In other types of brownout circuits the brownoutthreshold would typically be preset at around 80% of the nominal inputvoltage. Because the brownout threshold is set lower for brownoutdetection circuits that use the secondary side, these circuits increasethe likelihood that input conditions may exist where the normal lightingcircuits have failed but the emergency lighting has not started toprovide illumination.

As switch mode power converter technology has been improvedsignificantly over the past several years, it has now becomeeconomically feasible to replace the common transformers or capacitivedivider circuits used extensively in the past in emergency lightingsystems with a switch mode power converter. A switch mode powerconverter is an example of a wide input supply range converter. Thistype of circuitry offers the advantage of being able to operate over awide input supply range (85-305 VAC 50-60 Hz) that eliminates thevoltage specific taps needed with transformer or capacitive inputcircuits. The flexibility of this type of input circuitry offers manyadvantages over using transformers or capacitive dividers with eithertype of brownout detection described above. First, a switch mode powerconverter provides greater input range flexibility. Odd voltage andfrequency (eg. 220V 50 Hz) AC input requirements can be met withouthaving to specify different transformer types or capacitor values.Second, there is a reduction in the likelihood of field wiring mistakesthat can occur when an electrician selects the wrong tap to power thesystem. Third, using a switch mode power converter instead of atransformer or capacitive divider circuit may allow the size of theemergency lighting system to be reduced.

The conventional methods of brownout detection cannot be used when aswitch mode power converter is used in place of a transformer orcapacitive divider circuitry. This is the case because using the switchmode power converter circuitry eliminates key elements normally reliedupon to implement a simple, low cost, brownout detection circuit.

A first problem is that the set voltage tap is not available because ofthe wide input voltage range topology inherent in the switch mode powerconverter. Without this tap there is no common reference point that canbe used to generate the single voltage level that is proportional to theinput voltage regardless of the value of that input voltage.Accordingly, brownout detection circuits that rely on this set voltagetap cannot be used with a switch mode power converter.

Another problem is that the change in the secondary output voltage overthe wide range of input voltages is tightly regulated and therefore notuseful to brownout detection circuits. This means that brownoutdetection topologies that rely on secondary side outputs can no longerbe used to determine the change in the input voltage on the primary sideof the circuit in systems that utilize a switch mode power converter.

Another complication in implementing common brownout detectiontechniques with switch mode power converter technology results from thefrequent requirement in emergency lighting equipment to isolate theprimary and secondary sides of the circuit. Specifically, the brownoutdetection circuitry located on the primary side of the circuit mustcommunicate or send its output to the circuitry that controls the lampsor lighting on the isolated secondary side of the circuit.

SUMMARY OF THE INVENTION

The present invention comprises methods and systems for an emergencylighting unit that provides brownout detection and protection. Oneaspect of the present concept utilizes a switch mode power converter toproduce the low voltage DC for battery charging and control functionsprovided by emergency lighting systems. Another aspect of the presentinvention provides a brownout detection circuit capable of automaticallyadapting to a wide range of different input voltages. Another aspect ofthe present invention provides brownout circuitry that automaticallydetermines brownout threshold values based on the normal input voltageconnected to the system. These exemplary embodiments are mentioned notto limit or define the invention, but to provide an example ofembodiments of the invention to aid understanding thereof.

A first embodiment of the present invention is an emergency lightingunit with an input voltage interface for receiving an input voltage, abrownout detection component for detecting a brownout condition on theinput voltage and generating a brownout signal, a switch mode powerconverter for altering the input voltage, and an emergency lightingcontrol and battery charging component for controlling the charging of abattery pack and receiving the brownout signal.

A second embodiment of the present invention provides a method ofproviding emergency lighting by receiving an input voltage,automatically establishing a brownout threshold for the input voltage;and detecting a brownout condition. This method may be capable ofautomatically adapting to a wide range of different input voltages.Specifically, one embodiment is capable of maintaining brownoutthresholds for multiple nominal input voltages proportional to thosesame voltages. This embodiment includes a brownout detection circuitthat receives input voltages and establishes brownout thresholds forthose input voltages. For example, the system could be set to establisha brownout threshold of 80% of the normal input voltage value.Accordingly, if the normal input voltage is 120 VAC, then the embodimentwill set a brownout threshold of 96 VAC (80% of 120) and if the normalinput voltage is 277 VAC, then the system will set a brownout thresholdat 221.6 VAC (80% of 277). In this exemplary embodiment, the brownoutthreshold is 80% of the nominal input voltage. The inventioncontemplates setting at the brownout threshold at other proportionalvalues, i.e. 75% or 83%. This disclosure does not intent to in any waylimit the value or proportion of the voltage threshold. Variousthresholds will be appropriate for different circumstances and differentembodiments of this invention. There may be some cases were the brownoutthreshold would be set extremely low (for example at 1% of the nominalinput voltage) and other circumstances where it would be set highextremely high (for example at 99% of the nominal input voltage.) Theinvention also contemplates setting brownout thresholds that vary basedon nominal input voltage levels using a rule other than the proportionalrule mentioned above.

In this embodiment, after the system sets the brownout threshold, thesystem monitors the input voltage for a brownout condition. A brownoutcondition occurs when the input voltage drops below the brownoutthreshold. In this manner the system can detect a brownout condition. Inthis example, the brownout threshold is set at 80% of the nominal inputline value. If a brownout condition is detected, the system generates abrownout signal that is communicated in some manner to the equipment orcircuitry that controls the emergency light source. The method ofcommunicating this signal will vary depending on the requirements of theemergency lighting system. Although various methods of communicatingthis signal are discussed herein, the invention is not limited to anyparticular method.

A third embodiment of the present invention provides a brownoutprotection system with an establish brownout threshold component forautomatically determining a brownout threshold for an input voltage, amonitor input voltage component for detecting a brownout condition whenthe input voltage drops below the brownout threshold, and a signalemergency lighting component to signal emergency lighting when abrownout condition is detected.

A fourth embodiment of the present invention provides a method ofproviding emergency lighting by receiving an input voltage, monitoringthe input voltage for a brownout condition, if a brownout conditionexists, sending a brownout signal to activate emergency lighting; but ifa brownout condition does not exist, using a switch mode power converterto provide power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a system in accordance with oneembodiment of the present invention.

FIG. 2 illustrates a circuit diagram of a brownout detection componentin accordance with one embodiment of the present invention.

FIG. 3 illustrates a logic flow for performing brownout detectionfunctions in accordance with one embodiments of the present invention.

FIGS. 4 a-g illustrate a logic flow for performing brownout detectionfunctions in accordance with one embodiments of the present invention.

FIGS. 5 a-e illustrate a circuit diagram of a system in accordance withone embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring now to the drawings in which like numerals indicate likeelements throughout the several figures.

FIG. 1 illustrates a block diagram of a system in accordance with oneembodiment of the present invention. This figure shows a system dividedinto four major sections differentiated by fundamental functionaldifferences. As shown in the diagram, the input voltage interface 100receives an input voltage 108. Input voltage 108 is typically from thebuilding line voltage and in the range from 102 VAC to 305 VAC. Theinput voltage receiver 100 may also perform various rectifying,smoothing or other functions on the input voltage 108 ensuring thatgovernment standards for electrical equipment are satisfied. Once theinput voltage 108 has been rectified and smoothed by the input voltageinterface 100, the rectified input voltage 110 is provided for use inthe brownout detection 102 and switch mode power converter 104functions.

In FIG. 1, brownout detection is performed by monitoring the voltagelevel of the rectified input voltage 110. When the voltage level of therectified input voltage 110 is measured below a certain value, orbrownout threshold, a brownout condition is signaled. The brownoutthreshold may be hard-coded into the circuit, determined based on theinput voltage, or otherwise automatically determined based on some othercriteria. During brownout detection, the system monitors the rectifiedinput voltage 110 by repeatedly sampling the input voltage 110 andcomparing the voltage level with the brownout threshold. As long, as therectified input voltage remains above the brownout threshold, the systemdoes not need to signal a brownout. If the rectified input voltage 110drops below the preset brownout threshold a drive signal 112 isoutputted signaling for emergency lighting. The drive signal 112 may besent to a component or unit that is electrically isolated from thecomponent performing brownout detection 102. For example, this isolationfunction 116 could be accomplished using an optocoupler.

The brownout detection may occur by automatically detecting a brownoutthreshold. In this embodiment, brownout detection 102 is performed byautomatically establishing a brownout threshold 102 a, monitoring therectified input voltage 102 b, and signaling for emergency lighting 102c when a brownout lighting emergency occurs. The circuit performingthese functions determines a brownout threshold using the rectifiedinput voltage 110. For example, the brownout threshold may be set to 80%of the nominal value of the rectified input voltage 110. After thebrownout threshold is established, the system monitors the rectifiedinput voltage 110. As before, if the rectified input voltage 110 dropsbelow the preset brownout threshold a drive signal 112 is outputtedsignaling for emergency lighting.

In FIG. 1, the switch mode power converter 104 function converts therectified input voltage 110 into a charging voltage 114. The widely usednew family of universal input switch mode power supplies based on theTiny Switch-II off-line Switcher could perform such a function. Thebasic function of this circuitry is to provide a constant outputvoltage. Even though, the input voltage 108 and rectified input voltage110 may vary over a wide voltage range depending on the environment inwhich the system is installed, the switch mode power converter functionprovides a set voltage electricity for charging the battery. One exampleof the typical output voltage of the switch mode power converter 104 is19.2 V. This electrical signal is filtered and supplied to the chargercircuitry of the batter charging and lighting control 106 component.

The main purposes of the battery charging and lighting control 106component are to control the charging of the battery pack connected tothe unit and to enable the DC lamps when the brownout detectioncircuitry detects the input voltage levels are below the brownoutthreshold.

FIG. 2 shows a circuit diagram of one embodiment showing circuitry usedto perform brownout detection function 102. In FIG. 2, a rectified andfiltered input voltage 202 is provided to a first voltage dividerelement 204. In one embodiment, the voltage divider consists of 3resistors, shown as first voltage divider element 204, and one resistor,shown as second voltage divider element 208. The resistor network formedby the first voltage divider element 204 and the second voltage dividerelement 208 is sized so that the maximum voltage across second voltagedivider element 208 is below the maximum input limit of the brownoutcontroller.

The filter and bypass capacitors 206 are used to ensure that a pure DClevel and transient free sense signal is supplied directly to the inputof brownout controller 210 which performs the analog to digitalconversion. In one embodiment, the filter and bypass capacitors 206consist of an electrolytic capacitor and a ceramic capacitor.

In the brownout controller 210, the analog to digital conversion iscarried out using a microcontroller with an internal comparatorfunction, internal voltage reference and timers, and an externaldiscrete integrator. This technique is usually known as the Delta-Sigmaconverter. The benefits of this conversion technique include its highresolution and fast conversion speed. Other conversion options mayinclude, but are not limited to, various A/D converters withpost-processing logic devices to determine the appropriate action basedon the sampled A/D value. However, one embodiment uses a cost-effectivemicrocontroller with an internal EEPROM and comparator with multiple I/Opins.

The brownout controller 210 may be electrically connected to otheremergency lighting components in a multitude of ways. In the embodimentshown, the brownout controller 210 connects to a power supply via lineto power supply 214. The circuit calibration is initiated manually bypulling the GP5 input pin 218 to ground during the unit's power up. Themicrocontroller performs an auto-calibration provided that the inputreference voltage is stable and at the level that the microcontroller isdesigned to perform the calibration. One embodiment uses 120 VAC for thereference input line voltage. The calibration output 216 of the brownoutcontroller is used to signal when calibration is done. Calibration isdone after the brownout circuit has determined appropriate brownoutthreshold levels for each nominal line voltage that the circuit isintended to operate. The calibration output goes to high whenevercalibration is done, otherwise it stays low.

A significant advantage of the circuit configuration depicted in FIG. 2is its ability to recognize three different input voltages (120, 220-24and 277 VAC) and associate the appropriate brownout threshold specificfor the input voltage present. This circuit configuration is independentof the line frequency, so it may be used with 50 or 60 Hz systems.

The brownout controller 210 of the present embodiment utilizes anauto-calibration technique to compensate for the discrete componentstolerances. When the calibration mode is initiated, the brownoutcontroller 210 samples the input reference line voltage and based onthat value calculates and stores appropriate brownout thresholds foreach nominal line voltage. This is possible because the analog todigital conversion is linear. In one embodiment, the brownout thresholdis set to 80% of the reference input voltage sampled. Accordingly, withthis technique the brownout threshold remains proportional at 80% withany nominal input voltage, in the designated range.

It is important to sense an accurate input voltage to efficiently setthe brownout threshold. For example, if the preset brownout threshold isset to low, stray voltages on the neutral line could confuse thebrownout controller 210 and cause emergency lighting to remain off evenwhen a brownout condition has occurred. Conversely, if the brownoutthreshold is set too high, the brownout controller 210 may send a driveroutput turning the lamps on and causing the battery to discharge.

The present invention provides an accurate and reliable way of detectinga brownout state for different input voltage values. One embodiment ofthe present invention is designed for reliable operation from 85 VAC to305 VAC and to recognize three input voltages (120, 220-240 and 277VAC). However, the invention could easily be implemented for a differentinput voltage range or for different input voltage values. For example,one method of altering this embodiment to allow for higher AC linevoltages could be accomplished with the appropriate modification of thefirst voltage divider element 204 and the second voltage divider element208 and by using a higher resolution analog to digital converter.

FIG. 3 is flow chart illustrating an example of the logic that may beperformed using the microcontroller of the brownout controller 210. Theinvention contemplates using this or similar logic to determine abrownout threshold value and monitor for a brownout condition.

The processes begin at the start 302 block. Next, the brownoutcontroller 210 determines whether conditions are appropriate to set abrownout threshold 304. If the conditions are not appropriate, then thecontroller proceeds to block 310. However, if the conditions areappropriate, then prior to proceeding to block 310, the brownoutcontroller 210 determines an appropriate brownout threshold and outputsa signal indicating that calibration is complete as shown in blocks 306and 308.

In block 310, the brownout controller determines if the input voltage isless than the set brownout threshold. If the input voltage is not lessthan the brownout threshold, then the brownout controller outputs a nobrownout condition indication 312 and returns to block 310 to once againdetermine whether the input voltage is less than the brownout threshold.By looping in this manner, the input voltage is continually monitoredfor the existence of a brownout condition.

If the input voltage is ever determined to be less than the brownoutthreshold in block 310, then the brownout controller outputs a brownoutcondition indication 314 and proceeds to block 316. In block 316, thebrownout controller determines whether the input voltage is greater thanthe brownout threshold. If not, the brownout controller 310 returns toblock 314 again outputting a brownout condition indicator. By looping inthis manner, after a brownout has occurred, this embodiment continues tomonitor the input voltage.

If during a brownout, the brownout controller determines that the inputvoltage is greater than the brownout threshold in block 316, then thebrownout controller outputs a no brownout condition indicator 318 andreturns to block 310 to continue monitoring the restored input voltage.

FIG. 4 a is a flow chart illustrating another example of the logic thatmay be performed using the microcontroller of the brownout controller210. Note that FIG. 4 a represents only one of many possible ways toimplement the present invention that would be available to one ofordinary skill in the art.

FIGS. 4 b-4 g each illustrate a specific portion of the flow chart ofFIG. 4 a as described below. The exemplary logical flow shown in FIG. 4a recognizes three nominal input voltages (120, 220-240, and 277 VAC).Other embodiments of this invention include logic that recognizes othernominal input voltage values. For example, another embodiment of theinvention could include logic to determine thresholds for five differentnominal input voltages including voltage levels below and above thethree listed above.

For purposes of illustration, the flow chart of FIG. 4 a is broken downinto several logical areas. The initialization section 402 initializesvariables and outputs. The brownout threshold decision section 404determines whether brownout threshold calibration is required. Thebrownout calibration 406 section determines the appropriate brownoutthreshold value for each nominal input voltage. The input voltage readand compare section 408 reads the input voltage and sets variable valuesdepending on value of the input voltage. The monitor during non-brownoutout section 410 monitors the input voltage during non-brownoutconditions. Finally, the monitor during brownout section 412 monitorsthe input voltage during a brownout condition.

FIG. 4 b illustrates the initialization section 402 of FIG. 4 a, whichinitializes variables and outputs. The process begins with start element414. The SET CONFIG BITS, DEFINE EEPROM VARIABLES, DEFINE I/O PINS 416block sets the configuration bits, defines variables for theelectronically erasable programmable read only memory (“the EEPROM”),and defines the input output pins. The MAIN 418 block indicates thestart of the main logic. The DEFINE: VBROWN, VCHARGE, V120MAX, V220MAX,INPUT_VOLTAGE, DEBOUNC_COUNT 420 step defines variables.

The SET GP4 TO LOW, SET GP0 TO LOW 422 block sets two outputs of themicroprocessor to low. The GP4 output indicates when calibration of thebrownout threshold is done. When GP4 is high, calibration is done,otherwise it stay low. In this step, GP4 is set low. GP0 is the outputindicating when the lighting equipment should be turned on. GP0 maycorrespond to the driver signal 112 described above. When GP0 is high,the system is signaling for the emergency lighting to be illuminated,when GP0 is low, the system is signaling for the emergency lighting tobe turned off. The SET WDT TO 2.3 SECONDS 424 block sets the watch dogtimer to 2.3 seconds. This timer reset variable may vary significantlyin other embodiments depending on the intended application and functionof the system. The WAIT 4 SECONDS BEFORE DO ANYTHING 426 tells thesystem to wait for 4 seconds allowing the filtered DC signal tostabilize. This wait can also vary in length.

FIG. 4 c illustrates the calibration decision section 404 of FIG. 4 a,which determines whether brownout thresholds calibration is required.The calibration decision section 404 follows the initialization section402.

Steps 428, 430, 432, 434 and 436 repeatedly check the ground to ensurethat the calibration mode is desired. This technique prevents anyunintentional calibration in the field usage. Any unintentionalcalibration could cause false brownout thresholds and therefore couldcause a severe impairment of the emergency lighting unit.

Returning to the embodiment shown, in the i=1 428 block, the variable“i” is set to a value of 1. In the IS GP5=0? i++430 decision block, thevalue of input pin GP5 is checked to see if it is equal to 0. Thevariable “i” is also incremented in this step. The GP5 is a pull toground by user if the calibration mode is desirable. If GP5 is pulled toground then the variable “debounc_count” is incremented in block 432. IfGP5 is not equal to 0 then the variable “debounc_count” is set to zeroin block 434. In block 436, the microprocessor checks to see if thevariable “i” is equal to 105. If it is not, then the processing returnsto block 430 and GP5 is checked again. In this manner the “i” variableis used to create a loop that will repeat 104 times, because thevariable “i” is increased on every successive loop through step 430until “i”=105. In this manner, GP5 is monitored over a short period.Each time GP5 is checked, the variable debounc_count, which starts at 0,is either increased or reset to zero. Once the value of “i”=105, theprocessing continues to decision block 438.

In the IS DEBOUNC_COUNT>100 438 block, the variable is_debounc_count iscompared with 100. If the variable is greater than 100 then theprocessing proceeds to the calibration mode 406 processing. If it is 100or less the processing skips the calibration mode 406 processing stepsand proceeds directly to the input voltage read and compare section 408.The purpose of requiring over 100 successful pulls to ground insuccession is to ensure that the calibration mode is desired. The aboveprocedure is intended to reduce the chance of the microcontroller goinginto a calibration mode in the field usage.

FIG. 4 d illustrates the brownout calibration 406 section if FIG. 4 a,which determines the appropriate brownout threshold values for the inputvoltages. In the CALL A2D FUNCTION 440 block the A2D function is called.The input voltage is measured through the microcontroller's internalanalog to digital converter and the value is stored as v_calibrate. Inthe READ V_CALIBRATE AND STORE IN EEPROM 442 block the value of thev_calibrate variable is stored in EEPROM. In block 444, the v_calibrateis used to calculate the brownout threshold for each nominal linevoltage intended for this embodiment. In the SET GP4 HIGH 446 block theoutput GP4 is set to high. As described above GP4 is high whencalibration is complete. After calibration is complete, processingcontinues to block 408.

FIG. 4 e illustrates the input voltage read and compare section 408 ofFIG. 4 a which reads the input voltage and sets threshold variablevalues depending on value of the input voltage. In block 448,V120_MAX_ADDR is read and stored as V120Max and V220_MAX_ADDR is readand stored as V220Max. In block 450, function A2D is called and theresult is stored as input_voltage. Blocks 452 and 454 determine whichvalues to store as vbrown and vcharge. If the input voltage is less thanV120Max then in block 456 Vbrown is set to the value in VBROWN_120_ADDRand Vcharge is set to VCHARGE_120_ADDR. If the input voltage is not lessthan V120Max but less than V220Max then in block 458 Vbrown is set tothe value in VBROWN_220_ADDR and Vcharge is set to VCHARGE_220_ADDR.Finally, if the input voltage is not less than V120Max and is not lessthan V220Max, then in block 460 Vbrown is set to the value inVBROWN_277_ADDR and Vcharge is set to VCHARGE_277_ADDR. Once thesevariables are set processing continues to section 410.

FIG. 4 f illustrates the monitor during non-brownout out section 410 ofFIG. 4 a which monitors the input voltage during non-brownoutconditions. Blocks 462, 464, 466, and 468 perform integrity checks onthe values of Vbrown and Vcharge. If either the Vbrown or Vchargecontain the values that are below or above the valid range, then theVbrown and Vcharge are set to default values. In block 472, function A2Dis called and the return value is stored as input_voltage. In the SETGP0 TO LOW 474 block, GP0 is set to low. As described above, when GP0 islow the system is signaling for the emergency lighting to be turned off.In block 476, the system performs a brief wait.

In block 478, the input_voltage value is compared with the Vbrown valueto determine whether a brownout condition exists. According, Vbrown isthe brownout threshold. If the input_voltage is less than Vbrown thesystem loops back. In this way the system monitors the input_voltage byrepeatedly measuring it against the Vbrown brownout threshold value. Ifthe input_voltage does drop below Vbrown the system proceeds to section412.

FIG. 4 g illustrates the monitor during brownout section 412 of FIG. 4 awhich monitors the input voltage during a brownout condition. In block480, the system sets the output GP0 to high. As described above, GP0 isthe output indicating when the lighting equipment should be turned on.GP0 may correspond to the driver signal 112 described above. When GP0 ishigh, the system is signaling for the emergency lighting to beilluminated, when GP0 is low, the system is signaling for the emergencylighting to be turned off. In block 482, the system waits a shortperiod.

In block 484, the system again calls the A2D function and stores thereturn as input_voltage. In block 486, the input_voltage value iscompared with the Vcharge value to determine whether a brownoutcondition still exists. According, Vcharge is a threshold level that isabove the Vbrown brownout threshold level and is used to ensure that theline voltage is sufficient to sustain the charger circuitry. If theinput_voltage is lower than the Vcharge value the system loops back to480. In this way the system monitors the input_voltage during a brownoutby repeatedly comparing it against the Vcharge value. If theinput_voltage does rise above Vcharge, the system sets GP0 to low inblock 488, waits a short amount of time in block 490, then returns tosection 410, which monitors the input voltage during non-brownoutconditions.

FIG. 5 a-e illustrate a circuit diagram of one embodiment of the presentinvention. The circuit shown in FIG. 5 a is comprised of essentiallyfour functional components each of which is separately shown in FIGS. 5b-5 e. These four components also correspond to the four functioncomponents shown in FIG. 1: an input voltage interface 100, a switchmode power converter 104, a brownout detection 102 component ,and abattery charging and lighting control 106 component.

FIG. 5 b illustrates the input voltage interface 100, which providesrectification and filtering on the input voltage. The input voltageinterface 100 of this embodiment is composed of the AC line voltageconnecter 502 which is rated for 277 Vrms, a fusible, flameproofresister 504, four bridge rectifier diodes 506, rated 600V peakrepetitive reverse voltage, and a π filter 508. The AC input isrectified with the bridge rectifier diodes 506 and smoothed with the πfilter 508. In addition, the fusible resistor 504 combined with the πfilter 508 allows the system to meet FCC class B conducted emissionstandard EN550022 B (CISPR22 B).

FIG. 5 c illustrates the brownout detection 102 component of FIG. 5 aand is similar to the brownout detection component shown in FIG. 2 anddescribed above. The circuitry provides an accurate and reliable way ofdetecting a brownout states for multiple nominal input voltages. If abrownout condition is detected, the brownout detection 102 componentsends a drive signal to an optocoupler 510. The transistor side of theoptocoupler receives this signal. The use of optocoupler 510 allows thebrownout detection component to be electrically isolated from thebattery charging and lighting control 106 component.

FIG. 5 d illustrates the use of a switch mode power converter 104 in anemergency lighting system capable of brownout detection. Specifically,the circuitry illustrates the widely used new family universal inputflyback type switch mode power supplies. Unlike the PWM controller, thisdevise uses a simple ON/OFF feedback control to regulate the outputvoltage while the input rectified and filtered voltage can vary over awide range from 85 VAC to 305 VAC. The output voltage of this regulatoris based on the nominal battery voltage, battery cell chemistry,charging technique algorithm and the charger controller. One of thetypical output voltages of this power supply is set to 19.2V. Thissignal is filtered with an electrolytic capacitor and supplied to thebattery charging and lighting control 106 component.

FIG. 5 e illustrates a battery charging and lighting control 106component that may be a part of an emergency lighting system. The mainpurposes of this circuitry are to control the charging of the batterypack and to enable a DC lamp when the brownout detection 104 componentdetects a brownout condition. In FIG. 5 e, the schematic illustrates a12 Volt charger that can be configured for Lead-acid and NiCd batterypacks.

In one embodiment shown in FIG. 5 e circuit incorporates a push buttonswitch, referred to as a remote test interface component, that providesa way for users to test the emergency lighting system during normaloperation (non brownout conditions). The battery charging and lightingcontrol 106 shown in FIG. 5 also includes various connectors to simplifyuse of the system and to provide universality of the system. Forexample, the system includes battery connectors, lamp connectors,external lamp connectors, and remote test capability.

While this invention has been described in detail with particularreference to preferred embodiments thereof, it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention as described above and as defined in the appendedclaims.

1. A method of determining a brownout threshold for use in providingemergency lighting comprising: providing a circuit configured to operateover a wide range of input voltages; connecting the circuit to an inputvoltage with an expected voltage level, wherein the expected voltagelevel is a discrete voltage level within the wide range of voltages;sampling an actual voltage level of the input voltage; and in responseto sampling the actual input voltage level, automatically establishing abrownout threshold based on the sampled input voltage level and notbased on the expected voltage level.
 2. The method of claim 1 furthercomprising transforming the input voltage using a switch mode powerconverter.
 3. The method of claim 1 further comprising transforming theinput voltage to charge a battery.
 4. The method of claim 1 furthercomprising altering the input voltage to power an emergency lightingcontrol component.
 5. The method of claim 1 further comprising detectinga brownout condition using the brownout threshold.
 6. The method ofclaim 1 wherein the brownout threshold is proportional to the inputvoltage.
 7. The method of claim 1 wherein the brownout threshold isdetermined independent of the frequency of the AC line input.
 8. Themethod of claim 1 further comprising sending a brownout signal toactivate a lighting device.
 9. The method of claim 1 further comprisingactivating an isolated lighting device.
 10. The method of claim 1wherein receiving an input voltage comprises receiving an input voltageis in the range from 85 to 305 Volts.
 11. A method of providingemergency lighting comprising: receiving an input voltage; determining abrownout condition threshold voltage based at least in part on an actuallevel of the input voltage; monitoring the input voltage for a brownoutcondition, wherein a brownout condition is determined to exist when theinput voltage drops below the brownout condition threshold voltage; if abrownout condition exists, sending a brownout signal to activateemergency lighting; and if a brownout condition does not exist, using aswitch mode power converter to provide power.
 12. The method of claim 11wherein the switch mode power converter provides power to charge abattery.
 13. The method of claim 11 wherein the switch mode powerconverter supplies power to charge an emergency lighting controlcomponent.
 14. An emergency lighting unit comprising: a convertercapable of receiving a wide range of input voltages; and a brownoutdetection component capable of detecting a brownout condition bycomparing the input voltage with a brownout threshold value determinedusing a sample actual voltage level of the input voltage.
 15. The systemof claim 14 wherein the converter is a switch mode power converter. 16.The system of claim 14 wherein the converter operates over a rangebetween 85 and 305 VAC.
 17. The system of claim 14 wherein the converterdoes not have voltage specific taps.
 18. The system of claim 14 whereinthe converter accepts one or more odd voltage and frequency inputs. 19.The system of claim 14 wherein the brownout detection componentdetermines one or more brownout thresholds.
 20. The system of claim 14wherein the brownout detection component determines the one or morebrownout thresholds using the wide range of input voltages.